1. Field of the Invention
The method of the present invention relates to oil and gas production. More particularly, the present invention relates to a method of providing a durable sealant to prevent communication through the annular space formed between a drilled well bore and an inserted casing string in order to isolate the producing formation from all other formations exposed by drilling the hole.
Furthermore, the problem addressed by this invention is long-term sealing for zone isolation in petroleum wells. The need for a durable sealant to prevent communication through the annular space formed between a drilled well bore and an inserted casing string is driven by the need to isolate the producing formation that is targeted during the well drilling process from all other formations exposed by drilling the hole. In addition, the purpose is to isolate all formations that are drilled through from all other formations. An example is isolating potable water zones from brackish or salt water zones, and obviously from the producing formation. This isolation guarantees that hydrocarbons produced from the formation must flow into the casing and not around it. Additionally, zone isolation ensures that treatment fluids pumped into the formation to stimulate it will not flow into the annulus. This seal must withstand thermal and hydraulic stresses imposed on it throughout the well's life. The magnitudes of these stresses caused by well intervention, hydraulic fracturing, continued drilling below the casing, etc. can be significantly higher than one would imagine: on the order of several thousand psi. Additionally stresses can be either compressive or tensile.
2. General Background of the Present Invention
Hydraulic cement is routinely used as a sealant for oil and gas wells. While the composition seems to be ideal for the application since it is fluid for placement and then sets to a hard solid, set hydraulic cement has very low tensile strength. Most of the stresses generated on the annular sealant are tensile and of magnitude sufficient to create cement sheath failure.
The term “hydraulic cement” is meant to encompass any inorganic cement that hardens or sets underwater. Hydraulic cements, for example, include Portland cement, aluminous and pozzolan cement, and the like. The term “hydraulic cement” is also intended to include cements having minor amounts of extenders such as bentonite, gilsonite, and also is intended to include cements used either without any appreciable sand or aggregate material or such cements and admixed with a granular filling material such as sand, ground limestone, and the like. Thus, for example, any of the Class “A-H” and “J” cements as listed in the “ANSI/API Recommended Practice 10B-2 First Edition, July 2005” are suitable for this purpose.
This failure of cement seal in an annulus of a well previously sealed is documented routinely in the industry. Stress is induced by thermal cycling, pressure cycling, ceasing or re-initiating production, etc. Tensile hoop stresses imposed during these operations are easily sufficient in magnitude to cause seal failure.
Extensive prior art exists. Expansive reaction of MgO in cement is documented. Four patents are cited here as reference.
PAT. NO.TITLE4,797,159Expandable Cement Composition5,718,292Inflation Packer Method and Apparatus5,942,031Expanding Additive For Cement Composition7,036,586Methods of Cementing in Subterranean FormationsUsing Crack resistant Cement Compositions
Expansion is noted, but pre-stressing effects are not realized since concentration is insufficient. It is believed that the concentration range claimed is the reason pre-stressing occurs and results in improved seal performance.
Many other references to MgO as a cement additive are specific for formulation of MgOCl cement. This material is acid-soluble but offers no benefit in the way of pre-stressing the cement matrix.
The following illustrations represent the stresses imposed on an element of cement in the cement sheath, and the effective stress state with Pre-Stressed Cement.